python 3.x - local variable referenced before assignment (list)

Question

I encountered an interesting error. When defining a list outside the scope of a function, like below, it causes an error: UnboundLocalError: local variable 'a' referenced before assignment

def main():
    a = []
    
    def sub():
        a += ["hello"]
        return a 
    sub()
main()

However, with the same logic, if used a.append no error is raised

def main():
    a = []
    
    def sub():
        a.append("hello")
        return a 
    sub()
main()

Is there any reason for this?

Thanks

question from:https://stackoverflow.com/questions/65861664/local-variable-referenced-before-assignment-list

Answer 1

You have to use the nonlocal keyword in this case.
nonlocal documentation

def main():
    a = []
    
    def sub():
        nonlocal a
        a += ["hello"]
        return a 
    sub()
main()